If 10.57 g of magnesium reacts completely with 6.96 g of oxygen, what is the percent by mass of oxygen in magnesium oxide? Round to the nearest tenth of a percent.

39.7 %

Explanation:

magnesium + oxygen ⟶ magnesium oxide

10.57 g 6.96 g 17.53 g

According to the Law of Conservation of Mass, the mass of the product must equal the total mass of the reactants.

Mass of MgO = 10.57 + 6.96

Mass of MgO = 17.53 g

The formula for mass percent is

% by mass = Mass of component/Total mass * 100 %

In this case,

% O = mass of O/mass of MgO * 100 %

Mass of O = 6.96 g

Mass of MgO = 17.53 g

% O = 6.96/17.53 * 100

% O = 0.3970 * 100

% O = 39.7 %